1. Field of the Invention
This invention relates to a synchronous multiplexing system wherein the place of the head of a data signal to be multiplexed with a frame is indicated by a pointer (numeric value), and more particularly to a pointer re-setting method and apparatus thereof when converting a transmission line frame into an in-apparatus frame.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example of a conventional pointer re-setting apparatus, and FIG. 2 is a timing chart illustrating operation of the pointer re-setting circuit shown in FIG. 1.
Frame counter 1 counts the contents of the received frame on the transmission line side according to input frame pulse signal (frame synchronizing signal) 102 and clock signal 103 and outputs frame count values 106.
In response to frame count values 106 of the input frame counter, decoder circuit 2 outputs pulse signal 107 indicating the position of a pointer in input data signal 101 (here, count value n 30 1). Retiming circuit 3 retimes (determines and sets a point of time at which a pulse is present) input data signal 101 with pulse signal 107 from decoder circuit 2 to produce pointer value 108, and signal 109 is produced from calculation circuit 4 calculating frame count values 106 with pointer value 108. Signal 109 represents the head place of the data in data signal 101.
Data signal 101 and date head position signal 109 are written into temporary storage circuit 5 with clock signal 103 of the transmission line and then read out from buffer 5 with in-apparatus clock 105.
Second frame counter 6 counts in-apparatus side frames in response to in-apparatus frame pulse signal (in-apparatus frame synchronising signal) 104 and in-apparatus clock 105 inputted thereto and outputs frame count values 112. Further, second decoder circuit 7 produces and outputs pulse signal 113 indicating the place in which the pointer is inserted in the in-apparatus frame.
Retiming circuit 8 latches the frame count value of second frame counter 6 in response to head data indication signal 111 read out from buffer 5 and outputs in-apparatus pointer value 123 from count value 112 of in-apparatus side frames. Pointer value insertion circuit 12 to which the pointer value is inputted multiplexes the pointer value with read out data signal 110 from buffer 5 and outputs a resulted signal as output 126.
Meanwhile, a protection circuit is provided for renewing the in-apparatus pointer, and when the pointer value is renewed, the pointer value must not be changed for succeeding three frames. To this end, pointer value 124 of the last frame is held in timing circuit 9, and when non-coincidence is detected by incoincidence detection circuit 10, incoincidence detection signal 125 is outputted from incoincidence detection circuit 10. In response to incoincidence detection signal 125, pointer value renewal inhibition signal generation circuit 11 generates signal 121 for inhibiting renewal of the pointer value in order to stop renewal of the pointer value.
In the conventional pointer re-setting circuit described above, since the pointer is renewed in response to a data head-indication signal produced on the transmission line side, when the pointer value of the receive data signal of the transmission line varies for each four frames, renewal of the in-apparatus side pointer value is sometimes delayed. This state is illustrated in FIG. 3. In the example thus illustrated, an actual operation of the pointer in the transmission line to vary from a to b and then to c. Here, a', b' and c' denote in-apparatus pointer values corresponding to pointers a, b and c of the transmission line. When data head indication signal 111 is not generated between two positions of the in-apparatus pointer as indicated by A in FIG. 3, renewal of the in-apparatus pointer is delayed (as seen from B in FIG. 3, the in-apparatus pointer is renewed to c' when signal 121 next varies to the L level).
In this manner, in the conventional pointer resetting apparatus, also the inside of the apparatus operates in response to the phase of a frame of the transmission line. Accordingly, the conventional pointer re-setting circuit has a problem in that the pointer value after re-setting may not possibly be renewed immediately.